The properties of a number of enzymes occurring in the endoplasmic reticular membranes of mammalian liver and kidney are being studied and compared with corresponding enzymes of marine invertebrates. Levels of the activities of glucose-6-phosphatase, PPi-glucose phosphotransferase, NADPH-cytochrome c reductase, nucleoside diphosphatase and S-palmitoyl-coenzyme A:sn-glycerol-3-phosphate acyltransferase, studied as a function of the age of the animals, have revealed differing patterns of change during the neonatal and weaning periods, indicating that enzyme development is more closely related to the changing metabolic needs of the animal than to membrane proliferation. Differing latency properties of some of these enzymes are being studied as clues to enzyme location on or within the structure of the membrane. BIBLIOGRAPHIC REFERENCE: Stetten, M.R. and Goldsmith, P.K.: A Limulus glucose-6-phosphatase with phosphotransferase activity characteristic of vertebrate liver microsomes. Its possible evolutionary significance. Biochim. Biophys. Acta 444: 835-852, 1976.